1. Field of the Invention
This invention relates to a liquid crystal optical modulator, particularly to a transmission type of liquid crystal display device or liquid crystal optical modulator utilizing alignment control of liquid crystal molecules.
2. Related Background Art
In the prior art, techniques utilizing combination of diffraction grating with liquid crystal have been known for several different purposes.
For example, a device having grooves with periodical regularity formed on the surface of a substrate and provided with a liquid crystal thereon is known to have ability for aligning liquid crystal. Since this device is not specially intended to function as a diffraction grating, the grooves are made extremely shallow.
It is also known to form minute gratings from materials having different reflection characteristics and combine them with a liquid crystal for utilizing the polarizing function of the gratings. In this case, also, the thickness of the grating itself is not emphasized.
Further, a phase diffraction grating is known in which the grating are formed with a transparent member, and liquid crystal is disposed in the grooves on the grating. For example, in Japanese Patent Publication No. 3928/1978 and U.S. Pat. No. 4,251,137, it is disclosed as a display device or a variable color subtraction filter device. However, the device disclosed in Japanese Patent Publication No. 3928/1978 is a device for exhibiting mere decorative effect and not satisfactory as a display element for displaying letters or images, or as an optical modulating element for effecting transmission or interception of light flux.
On the other hand, the variable color subtraction filter element disclosed in U.S. Pat. No. 4,251,137 utilizes the change in diffraction effect caused by the change in refractive index difference between the grating and the liquid crystal for the light transmitting through the cell at a definite angle by varying the direction of the liquid crystal arranged between the diffraction gratings by action of an electrical field. Firstly, however, this element involves a technical difficulty in preparation, and secondly it has a drawback of unsatisfactory characteristics in actuation.
Even when a relatively large .DELTA.n (.DELTA.n: refractive index difference) may be used with an actually available liquid crystal, for achieving sufficient diffraction effect, gratings with greater depth of groove relative to the grating pitch must be formed. Particularly, while formation of a grating having a depth and pitch of 3 .mu.m or less is optically effective, working technique for producing gratings with such a size requires the most advanced technique in semiconductor device, and such devices cannot easily produced.
Next, as the problem in actuation, the liquid crystal confined in such a deep groove not only receives planar constraint from both upper and lower surfaces of the substrate, but also receives strong constraint from the walls on both sides of the groove in the grating. This means that, while the longer axes of liquid crystal molecules will stably align in the groove direction. On the contrary, when they are desired to be in another aligned state by an external force, they will greatly resist against the force. This means that the initial aligning will not easily be destroyed by an external force, namely an electrical field applied within the cell, suggesting that sharp voltage transmission characteristics required for time division characteristic cannot easily be obtained.
It has been known in the prior art that, when threshold voltage value is exceeded by applying direct current on a liquid crystal cell, "Williams domain" is generated and, when the electrical field is intensified, the width or the pitch of the domain becomes smaller to give a diffraction grating. For example, it is disclosed in Soffer et al: "Optical computing with variable grating mode liquid crystal devices", Proc. SPIE, 1980, 218, p. 81.
The diffraction grating of this prior art undergoes change in grating pitch according to a change in voltage, and therefore spectral characteristics of diffracted light are changed. However, this diffraction grating is not beneficial in maintenance of constant diffraction conditions and time divisional driving in which a bias voltage is applied.